In the related art, diesel engines are installed on construction machines including hydraulic shovels, bulldozers, dump trucks, and wheel loaders.
FIG. 21 illustrates a configuration of a conventional construction machine 100. Referring to FIG. 21, the construction machine 100 uses an engine 2, which is a diesel engine, as a driving source to drive a hydraulic pump 3. A capacity variable type hydraulic pump is used as the hydraulic pump 3, and a tilt angle of an inclined plate 3a of the hydraulic pump 3 is varied to change a capacity q (cc/rev). Pressure oil discharged at a discharge pressure PRP and a flow rate Q (cc/min) from the hydraulic pump 3 are supplied to hydraulic actuators 31, 32, 33, 34, 35, and 36 including a boom cylinder 31 through operation valves 21, 22, 23, 24, 25, and 26. The operation valves 21, 22, 23, 24, 25, and 26 are operated by operating operation levers and 42. Pressure oil is supplied to each of the hydraulic actuators 31, 32, 33, 34, 35, and 36 to be driven, and then, a work device including a boom, an arm, and a bucket connected to the hydraulic actuators 31, 32, 33, 34, 35, and 36, a lower travel body, and an upper swing body are operated. While the construction machine 100 is operated, loads applied to the work device, the lower travel body, and the upper swing body is continually varied according to the quality of earth to be excavated, the slope of travel path. Accordingly, a load of the hydraulic device (the hydraulic pump 3), that is, a load applied to the engine 2 is varied.
An output P (horsepower; kw) of the engine 2 is controlled by adjusting a fuel amount injected into the cylinder. The adjusting of the fuel amount is performed by controlling a governor 4 provided to a fuel injection pump of the engine 1. An all speed control type governor is generally used as the governor 4. An engine speed n and a fuel injection amount (torque T) are adjusted according to a load to maintain a target engine speed set with a fuel dial. That is, the governor 4 increases or decreases the fuel injection amount such that the target speed is equal to the engine speed.
FIG. 22 is a torque graph of the engine 2 with a horizontal axis being the engine speed n (rpm; rev/min) and a vertical axis being the torque T (N.m). Referring to FIG. 22, a region defined as a maximum torque line R denotes the performance of the engine 2. The governor 4 controls the engine 2 to prevent the torque T from reaching an exhaust gas limit over the maximum torque line R, and prevent the engine speed n from reaching over rotation over a high idle speed nH. The output (horsepower) P of the engine 2 is maximal at a rated point V on the maximum torque line R. Along an iso horsepower curve J, horsepower absorbed at the hydraulic pump 3 is disposed.
When the maximum target speed is set with the fuel dial, the governor 4 adjusts speed on a maximum speed regulation line Fe connecting the rated point V to a high idle point nH.
As the load of the hydraulic pump 3 is increased, a matching point where the output of the engine 2 and a pump absorption horsepower are in equilibrium moves to the rated point V on the maximum speed regulation line Fe. When the matching point moves to the rated point V, the engine speed n is slowly decreased. The engine speed n is a rated speed at the rated point V.
As such, in the state the engine speed n is fixed at a substantially constant high speed, when a work is performed, fuel consumption rate is increased (deteriorate), and pump efficiency is decreased. The fuel consumption rate (hereinafter, fuel efficiency) means a fuel consumption amount per hour and output of 1 kw, which is an index indicating the efficiency the engine 2. In addition, the pump efficiency is an efficiency of the hydraulic pump 3 defined as volume efficiency and torque efficiency.
Referring to FIG. 22, an iso fuel efficiency curve M has a trough M1 where the fuel efficiency is minimal. The fuel efficiency is increased from the minimum fuel efficiency point M1 to the outside.
As illustrated in FIG. 22, the regulation line Fe corresponds to a region where the fuel efficiency is relatively large on the iso fuel efficiency curve M. Thus, according to a conventional control method, the fuel efficiency and the engine efficiency are poor.
In the case of the capacity variable type hydraulic pump 3, when the discharge pressure PRP is constant, as the pump capacity q (the tilt angle of the inclined plate) is increased, the volume efficiency and the torque efficiency are increased, so that the pump efficiency is high.
Referring to Formula 1, in the state where the flow rate Q of pressure oil discharged from the hydraulic pump 3 is constant, when the speed n of the engine 2 is decreased, the pump capacity q can be increased. Thus, when the speed of the engine 2 is decreased, the pump efficiency can be increased.Q=n·q  (1)
Thus, to increase the efficiency of the hydraulic pump 3, the engine 2 is operated in a low speed region where the speed n of the engine 2 is small.
However, as illustrated in FIG. 22, the regulation line Fe corresponds to the high speed region of the engine 2. Thus, according to a conventional control method, the pump efficiency is low.
In addition, when the engine 2 is operated on the regulation line Fe, the engine speed is decreased at a high load state. Thus, engine stop may occur.
A control method of substantially fixing an engine speed regardless of the load is described above. On the other hand, a control method in which an engine speed is varied according to a lever operation amount and a load is disclosed in Patent Document 1.
In Patent Document 1, as illustrated in FIG. 22, an target engine driving line L0 passing through a fuel efficiency minimum point M1 is set.
In addition, a necessary speed of the hydraulic pump 3 is calculated based on operation amounts of the operation levers 41, 42, 43, and 44, and a first engine necessary speed corresponding to the necessary speed of the hydraulic pump 3 is calculated. Furthermore, an engine necessary horsepower is calculated based on operation amounts of the operation levers 41, 42, 43, and 44, and a second engine necessary speed corresponding to the engine necessary horsepower is calculated. In this case, the second engine necessary speed is calculated as the engine speed on the target engine driving line L0 of FIG. 22. The engine speed and the engine torque are controlled to obtain the greater one of the first and second engine necessary speeds.
As illustrated in FIG. 22, when the speed of the engine 2 is controlled along the target engine driving line L0, fuel efficiency, engine efficiency, and pump efficiency are improved. This is because, even when an identical horsepower is output to obtain an identical required flow rate, matching with a point pt2 on the iso horsepower line J and the target engine driving line L0 is adapted for a move from a high speed and a lower torque to a low speed and a high torque for increasing the pump capacity q and a driving to the fuel efficiency minimum point M1 on the iso fuel efficiency M, relative to matching with a point pt1 on the regulation line Fe. In addition, since the engine 2 is driven in a low rotation region, noises, engine friction, and pump unload loss are reduced.
In addition, in the construction machine field, as construction machines using a hybrid manner in which the driving force of an engine is assisted by a generator motor are developed, many patents have been applied.
For example, in Patent Document 2, as illustrated in FIG. 22, the engine 2 is controlled along a regulation line Fe0 corresponding to a set speed set with the fuel dial. An target speed nr corresponding to a point A where the regulation line Fe0 crosses the target engine driving line L0 is determined. When a deviation between the engine target speed nr and the current engine speed n is plus, a generator motor performs electric motor action to assist the driving force of the engine 2 using torque generated from the generator motor. When the deviation is minus, the generator motor performs generation action to generate electricity to store power in a storage battery.    [Patent Document 1] Japanese Patent Application Laid-Open (JP-A) No. 11-2144    [Patent Document 2] Japanese Patent Application Laid-Open (JP-A) No. 2003-28071